By definition, a retaining wall is designed to retain dirt. The dirt exerts an outward pressure on the wall, which the wall must be able to withstand. The taller the wall, the more pressure the dirt applies to the wall, so the more pressure the wall must be able to withstand. The ability of the wall to withstand pressure from the dirt is tied directly to the effective mass of the wall. The taller the wall, the more mass is required.
A common way to build tall walls is to use very massive blocks. The blocks may be natural boulders, but more commonly are prefabricated concrete blocks. If made of concrete, they often have mechanisms to interlock with adjacent blocks, and may have a surface formed to look like smaller blocks.
This approach works, but has the disadvantage that the blocks normally must be put in place using heavy construction equipment. That means the heavy construction equipment must be able to reach to location in which the blocks will be placed and requires skill in using the equipment, both of which limit the range of potential uses and add to the cost of building the wall. In addition, the scale is aesthetically inappropriate for smaller landscaping applications.
A second approach used both with massive blocks and smaller blocks suitable for landscaping is to use a geogrid to turn the mass of part of the dirt into part of the effective mass of the wall. A geogrid is a sheet of fabric or mesh, typically of a plastic polymer or metal, which is placed between rows of blocks in the wall as the wall is built, and extends back into the dirt. An example is shown in U.S. Pat. No. 6,447,211 (Scales et al.). To use it, the dirt is excavated back from where the wall will be and the first few rows of the wall are put in place. One end of the geogrid then is placed on top of the partial wall and laid out into the excavated area. Several more rows are built, and then the area on top of the geogrid is back-filled, typically with a layer of gravel next to the wall to provide drainage, and the excavated dirt further from the wall. A new layer of geogrid is laid down, and the process is repeated until the wall reaches the desired height. Variants on geogrid include wall anchors and wall nails, which are simply different structures to engage the dirt.
The advantage to this approach is that it turns the portion of the dirt which is between the geogrid layers into part of the effective mass of the wall. The disadvantage is that the dirt must be excavated quite far back—much farther than is necessary to build the wall itself. And the higher the wall, the farther back the excavation must go. Depending on the nature of the ground, this may be very difficult to do, and usually requires excavating equipment.
A third approach is to build a two-stage wall. In this approach, two stone walls are built adjacent to, but spaced from, each other. A connector of some sort (wood, stone, metal, plastic) is used to bridge the gap between the walls at intervals to stabilize the walls against each other, and the space between the walls then is filled with rubble. This approach dates back at least to the Middle Ages, and is how the curtain walls of most castles were built. It sometimes is referred to as a crypt construction.
In more modern construction, the two walls typically are built of prefabricated concrete blocks or slabs, and the connectors between them are usually formed of metal. Examples of this type of construction can be found in U.S. Pat. No. 6,802,675 (Timmons et al.) and U.S. Pat. No. 8,616,807 (Ogrochock). In this approach, reinforcing wire mesh of the type usually used to reinforce concrete pavement is attached to the inner wall. Links are connected to the outer wall, and then it all is wired together. The space between the walls is filled with gravel, rock or concrete.
The advantages to this construction technique are that it uses the same techniques and equipment as typical highway construction, that it can extend to considerable heights and hold back the consequent substantial pressures. Relatively little excavation is required behind the wall—just the amount needed to be filled with gravel to provide drainage. The disadvantages to this construction technique are that the materials are relatively expensive, and it is very labor intensive, since each of the connectors must be wired or bolted into place between the walls. As a result, it is primarily used in highway construction, where the advantages strongly outweigh the disadvantages.
Another approach designed for use in construction of smaller walls is shown in US2012/073229 (Castonguay et al.) and U.S. Pat. No. 5,845,448 (Potvin). In this approach, the blocks have keyhole slots in them into which the ends of a connector are inserted to hold the blocks in the two walls in position, or the blocks have protrusions around which the connector ends fit. The connectors may have interconnections, to make them longer or to connect them cross-wise. The connectors preferably are formed of plastic. The space between the walls then is filled with gravel or rock. This structure can be used to form a two-stage retaining wall, and can also be used to build a stand-alone wall wider than the width of the blocks used to build it.
The primary advantage to this approach is that it can be used with smaller blocks, suitable for placement by hand. This dramatically expands the flexibility and range of use of the system, for example, it can easily be used for landscaping without the use of heavy construction equipment. The disadvantage of this approach is that it requires specialized blocks with keyhole slots or protrusions, which adds considerably to the cost of the blocks.